Study On Bed Load Transport Based On Characteristics Of Bidirectional Position Of Sediment

As one of the main moving forms of sediment in river,the study of bed load movement is of great significance in river dynamics theory and practical engineering.In order to further analyze the law of uniform bed load transport,the bidirectional position characteristics of sediment on the bed surface are measured and analyzed in this paper,and the distribution law of the bidirectional relative position of sediment is obtained.The entrainment probability,the threshold of uniform sediment and bed load transport rate are studied on the basis of the law of bidirectional relative position.The main contents and conclusions are as follows:(1)By measuring the bidirectional position parameters of the uniform glass sphere and the uniform river pebble on the two-dimensional bed,the distribution laws of the bidirectional position parameters of the uniform sediment on the two-dimensional are obtained through statistical analysis.The result shows that hiding angle and exposure angle of glass spheres obey normal distribution with same parameters,the relative protrusion degree of glass spheres and natural cobblestones are negative-skewed and the former asymmetry coefficient is larger than latter,the relative exposure degree of glass spheres and natural cobblestones are positive-skewed and normal respectively.In addition,the bidirectional hiding-exposure degree of glass spheres and natural cobblestones are both normal distribution,and the mean value and variance of the former are smaller than latter.(2)Natural river pebbles are used as experimental materials to explore the bidirectional position characteristics of river pebbles on two-dimensional beds with different size distribution.The result shows that the probability density distribution of relative protrusion degree of natural sediment follows negative skewed distribution,and with an increase of the standard deviation for pebbles the asymmetry coefficient of the relative protrusion degree increases gradually.Then,the probability density distribution of relative exposure degree follows normal distribution approximately,and with an increase of the standard deviation for pebbles the mean value of the relative exposure degree decreases moderately.Additionally,the probability density distribution of the bidirectional hiding-exposure degree follows normal distribution,too,and with an increase of the standard deviation for pebbles the mean value of the bidirectional hiding-exposure degree increases gradually.(3)Based on the bidirectional distribution of uniform spherical particles,the formula of critical incipient shear stress of uniform sediment is established.The force analysis of single particle is carried out,and the turbulence characteristics of water are combined.The threshold probability formula including the randomness of position and the randomness of flow turbulence is derived by means of rolling incipient motion.Next,according to the three threshold standards put forward by Dou Guoren,the variation trend and range of critical incipient shields sediment with bidirectional position angle under different threshold standards are determined.(4)Based on the bidirectional distribution law of uniform spherical particles,the formula of bed load sediment transport rate is constructed from different aspects.The force analysis of single particle in three different incipient modes,rolling,sliding and lifting,are analyzed,and the threshold probability formula of sediment in different modes is established.It is found that the rolling mode is the easiest and the lifting mode is the hardest to move.In this paper,the rolling threshold probability is introduced into the bed load transport rate formula,and a new bed load transport rate formula is constructed by comparing and analyzing the parameters such as the number of sediment particles on the unit bed and the moving velocity of the bed load.Then,the force analysis of the sediment on the slope is carried out,and the formula of bed load transport rate suitable for the slope bed was derived.In addition,based on the exchange theory among bed material,bed load and suspended load,and by referring to the properties of Markov chain,the state probability formula of sediment which is included in the bidirectional position relation of sediment is obtained,and finally the bed load transport rate formula based on Markov process is established.(5)The position characteristics of the sediment particles on the three-dimensional bed were preliminarily explored.The force analysis of the particles under three kinds of accumulation conditions is carried out,and the critical incipient velocity formula of the spherical particles is derived.